1. Field of the Invention
This invention is related to the remediation of contaminated aquifers. More specifically, the invention relates to the recovery of DNAPLs from contaminated aquifers by density modification displacement.
2. Description of Related Art
The widespread detection of organic contaminants in groundwater has prompted the initiation of remediation efforts at numerous sites throughout the United States. The most common technology employed for aquifer remediation is "pump-and-treat," in which contaminated groundwater is extracted from the subsurface via wells or drainage systems and then treated above ground. Frequently, this approach results in a rapid decrease in aqueous-phase concentrations of the contaminant and a reduction in the size of the contaminated groundwater plume. This initial success, however, is often followed by a gradual decline in contaminant concentrations which may persist for years or decades. Thus, the remediation of an aquifer to a specific health-based standard (e.g., aqueous- or solid-phase concentration) is often not achievable within an acceptable time frame. In addition, the costs associated with maintaining a pump-and-treat system, and monitoring contaminant levels in groundwater can be prohibitive.
One of the most recalcitrant remediation scenarios facing regulatory agencies involves the contamination of aquifers by organic compounds existing as a separate liquid phase or non-aqueous phase liquid (NAPL). Classic examples of NAPLs include chlorinated solvents, such as trichloroethylene (TCE), and petroleum-based products, such as gasoline. Upon entering the unsaturated zone, NAPLs migrate downward as a result of gravitational and capillary forces. If the volume of NAPL released to the subsurface is sufficient to reach the water table, an organic liquid that is more dense than water (DNAPL) will tend to travel vertically through the saturated zone until a confining or low permeability layer is reached. In contrast, a NAPL that is less dense than water (LNAPL) will tend to spread laterally within the capillary fringe, forming a "lens" of free product. Subsequent fluctuations in the water table, however, may result in vertical displacement of the LNAPL lens and consequent redistribution within the saturated zone. The overall distribution of a NAPL within an aquifer formation may be extremely complex due to the presence of small- and large-scale heterogeneities and variations in groundwater flow.
As the NAPL is transported through the subsurface, a portion of the organic liquid is retained within soil pores as ganglia or globules due to capillary forces. Under normal flow regimes the entrapped NAPL will not be displaced from the porous media. This immobile fraction of NAPL is commonly referred to as "residual NAPL" and may occupy between 5 and 40% of the pore volume. Due to the low solubility of most organic liquids, the entrapped NAPL frequently represents a long-term source of groundwater contamination. Thus, conventional pump-and-treat systems, which are based on NAPL dissolution into groundwater, have proven to be an ineffective and costly method of aquifer restoration. When NAPLs are present in the subsurface, the initial success of pump-and-treat systems can be attributed to removal of the contaminated groundwater plume, while the persistent low levels of contaminant in extraction wells is due to NAPL dissolution into "clean" groundwater passing through the zone of residual NAPL contamination.
To overcome the limitations associated with conventional pump-and-treat remediation systems, surfactants have been proposed as a means for enhancing the recovery of NAPLs from subsurface systems. Surfactant-based remediation technologies are based primarily on two recovery processes: (a) micellar solubilization of hydrophobic organic compounds and (b) displacement of the entrapped NAPL as free product due to reductions in the NAPL-water interfacial tension. As shown in FIG. 1, in practice, aqueous surfactant solutions would be introduced through an injection well, allowed to flow through the zone of NAPL contamination, and the resulting NAPL/surfactant solution would be recovered using a down-gradient extraction well.
The most common approach employed in surfactant-enhanced aquifer remediation capitalizes on the ability of surfactants to form micelles, which function to increase the aqueous solubility of NAPLs. Surfactants are amphiphilic compounds, possessing both hydrophilic and hydrophobic moieties. Below the critical micelle concentration (CMC), surfactant molecules exist as monomers and have a minimal effect on the aqueous solubility of most hydrophobic organic compounds (HOCs). As the surfactant concentration approaches the CMC, individual surfactant monomers tend to associate with one another to form micelles consisting of a hydrophobic core surrounded by a hydrophilic mantle or shell. Above the CMC, the solubility of hydrocarbons in aqueous surfactant solutions has been shown to increase in a linear fashion. The observed linear enhancement in solubility is attributed to the incorporation or partitioning of hydrocarbons within surfactant micelles. The resulting solution is thermodynamically stable, consisting of NAPL solubilized or dispersed within surfactant micelles, which are on the order 10 to 100 nm in diameter. This type of system is often referred to as a NAPL-in-water (NAPL/W) microemulsion or Winsor Type I system. In a conceptual sense, micellar solubilization is similar to the partitioning of dissolved organic contaminants into soil organic matter, although it should be recognized that soil organic matter represents a far less uniform and structured environment in comparison to a micellar solution. Over the past several years a number of research groups have demonstrated the capacity of surfactants to improve the removal of entrapped NAPLs from unconsolidated porous media using both micellar solubilization. However, this approach is likely to be prohibitively expensive due to the large amount of surfactant necessary to remove all the NAPL contaminant. A need, therefore, exists for an economically viable remediation process not requiring large amounts of surfactant.
The second approach employed in surfactant flushing is based on the tendency for surfactants to accumulate at interfaces, and the resulting decrease in interfacial tension between entrapped NAPL globules and aqueous surfactant solutions. For several NAPL-surfactant systems, reductions in the interfacial tension to values less than 1.0 dyne/cm have been associated with the onset of NAPL displacement during surfactant flushing. Almost complete displacement (99% recovery) of residual tetrachloroethylene (PCE) from several size fractions of Ottawa sand has been observed after flushing with only 1.5 pore volumes of surfactant solution. The same approach has been investigated by the petroleum industry to enhance oil recovery. In such applications, brines and/or alcohols are frequently added to the surfactant formulation in order to achieve ultra-low interfacial tensions (less than 0.001 dyne/cm).
Surfactant-induced displacement of NAPLs as free product has been shown to provide an efficient means of NAPL recovery in laboratory soil columns. For example, essentially all of the residual PCE was displaced from soil columns after injecting 1 pore volume of a surfactant solution which reduced the IFT to approximately 0.09 dyne/cm. In contrast, a surfactant solution which functioned as a solubilizing agent (i.e., the IFT was not low enough to induce mobilization) required approximately 20 pore volumes of flushing to reach achieve recovery efficiencies. Therefore, displacement of the NAPL phase as a free product is very attractive, both in terms of cost and time required for clean-up. However, utilization of this approach in the field could lead to the uncontrolled migration of the displaced free product. This is of particular concern for DNAPLs, which could travel downward through an aquifer formation until a confining layer was encountered. Thus, the displacement process could, in effect, create a NAPL distribution that was more difficult to remediate than the original spill.
In order to successfully employ the displacement process in the field, it is necessary to induce DNAPL displacement and control the subsequent migration of the free product.
It is therefore an object of the present invention to provide a method of groundwater remediation which is economically efficient.
It is a further object of the present invention to provide a method of NAPL displacement which does not lead to uncontrolled migration of the NAPLs.